This invention relates generally to aircraft engines and, more particularly, to fuel control systems for turbofan engines of the augmented type.
It has long been recognized that afterburning or duct burning turbofan engines are susceptible to rich instability conditions, sometimes called "rumble", during operation within certain portions of an air vehicle flight envelope. Rich instability is the pressure oscillation that occurs in the longitudinal mode of the burner wherein the amplitude may reach a level sufficient to cause structural damage. During these unstable conditions of operation, various undesirable conditions may result, such as, for example, stall, augmenter blow-out, or mechanical damage.
Rich instability differs in many respects from screech, which is a higher frequency transverse pressure oscillation which may occur in high pressure and high temperature conditions. In addition, while screech usually occurs at near stoichiometric fuel/air ratio, rich instability may reach unacceptable magnitudes at fuel/air ratios as low as 0.02.
It is recognized that at high altitudes and low Mach numbers, stable combustion cannot be maintained above a certain fuel/air ratio, commonly referred to as the rich stability limit. That is, for a particular flight condition, any further increase in altitude or decrease in Mach number will cause a reduction in the rich stability limit. In order to avoid the heretofore mentioned instability conditions which may cause mechanical difficulties and damage to the engine, it is necessary to maintain the fuel/air ratio to a point below the rich stability limits.
It is therefore an object of the present invention to maintain a stable combustion condition in an augmented turbofan engine.
Another object of the present invention is to maintain a fuel/air ratio of a turbofan engine at a level which does not allow a rich instability condition to exist.
Yet another object of the present invention is the provision for operating an augmented turbofan engine in high altitude and low Mach number conditions without encountering unstable burner conditions.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.